StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>Imagine having a GUI in your application. Most of the time, control flow wouldn't be very linear, i.e. instead of having a clear sequence of actions you'd have a user that interacts with the GUI (like buttons, menus etc.).</p> <p>This is essentially an event driven model which can be implemented quite nicely with the signals and slots pattern. signals are events which are generated by objects (think GUI components) and slots are the receivers of those events.</p> <p>Here is an example: imagine you have a checkbox, represented as an object in your programming language. Multiple things can happen to that checkbox: it can be toggled, which in turn also means that it's either set or unset. Those are the signals which it can emit. We will name them checkboxToggled, checkboxSet and checkboxUnset. As you see, in this example, the checkbox will always emit the checkboxToggled signal when toggled, but also exactly one of the two other signals, depending on how the state changes.</p> <p>Now imagine having some other objects, namely a label, which for the sake of this example always exists as an object but can "appear" and "disappear" and a system beep (also represented by an object), which can simply beep. Those are the slots those objects have. We will call them "messageAppear", "messageDisappear" and "beep".</p> <p>Suppose you want the system beep to beep everytime the checkbox is toggled, and the label to appear or disappear depending on whether the user checked or cleared the checkbox.</p> <p>You would thus connect the following signals to the following slots (signals on the left, slots on the right):</p> <pre><code>checkboxToggled -> beep checkboxSet -> messageAppear checkboxUnset -> messageDisappear </code></pre> <p>That's basically it.</p> <p>signals and slots may also have arguments. For example, using a slider which sets a numerical value, you would like to send the changed value along with the emitted signal as soon as the user moved the slider: sliderChanged(int).</p> <p>Of course, to actually do something useful you would write some own classes which would contain some own signals and slots. This is done quite easily and using these own signals and slots, you have a nice way to interact with the GUI or other parts of your code in an event driven manner.</p> <p>Keep in mind that signals and slots are often symmetric in the sense that there may often be a signal corresponding to a slot. For example, a checkbox may emit a signal when toggled, but it may also contain a slot that toggles the checkbox itself. It would be easy to implement to separate checkboxes that are always set oppositely to each other.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload